For Civ6 and Civ7 (presumably), Firaxis and 2K want the games to be playable not only Windows PCs and Macs but also on the leading consoles and tablet devices. That involves a great deal more complexity in testing as well as design decisions for CPU architectures
Firaxis has greatly increased the amount of work required to release a new game in the franchise.
While I can see that Firaxis might consider some design decisions for Civ7 due to Consoles and Mobile, I don't think that when designing Civ6, and its DLCs, the Devs took into account the Consoles. Civ6 is much more Complex than Civ5, where you would think that the latter would have been more successful on console because of that.
Besides, It's not Firaxis that worked on the Console versions, in Fact, all non-Windows versions (including Mac and Linux) were developed by Aspyr AFAIK. And as we know, the Mobile and Console Versions seem to have a different coding system than the PC version, especially on the UI side.
So I wouldn't say that Firaxis is taking longer to work on a new Civ Game because of the different Platforms they want the Game to be released on. I'm sure that the Firaxis Civ Dev Team are solely focusing on designing the Game as usual (I hope so at least), and intrusting the OS, Linus, Console and Mobile Ports to a 3rd Party. It wouldn't be feasible for Firaxis to take all that burden on their shoulders, and it's not necessary. After all, we have seen many complex Games that were developed for PC, and then later on ported to other Platforms, successfully. The Only Issue with that that we have with Civ6, is that 2K doesn't seem to hire Aspyr often enough to make Updates for those Platforms. However, even for the PC version, any QA/internal testings that are done to Civ6 weren't enough to fix the Game, most testing is done by the Players and them Reporting any Bugs/Imbalances. And yet, we are still here and waiting for that final BugFix Patch that will fix the Game crashes, the Asset Bug Limit, the crazy Monopoly Tourism Bug,...etc.
Though, what I would like for Civ7 to consider for Consoles and Mobile, is making it possible to mod them.